Hmm... Gotta go with the .30 Rem. AR. It never had a real chance at being successful, unfortunately, due to premature claims of velocity, boneheaded marketing decisions, and good ol' fashioned poor timing (overshadowed by .300 Blackout soon after it was finally ready for market.) It's not perfect, however it is a very fine hunting cartridge and is marvelously accurate.
Other contenders for me are the .450 Bushmaster and .35 Remington.